In recent years, in order to reduce the weight of automobiles, attempts to increase the strength of steel sheets have been performed. In general, increasing the strength of the steel sheet leads to a deterioration of the formability such as a hole expansibility, and thinning the sheet thickness for weight reduction leads to a decrease in fatigue life. Accordingly, in order to develop a steel sheet which shows the high strength and which enables the weight reduction of automobiles, it is important to achieve improvements in the formability such as the hole expansibility and in the fatigue properties in addition to the increase in the strength of the steel sheet.
Conventionally, it is known that an excellent fatigue life can be obtained by producing steel which has composite structure consisting of ferrite and martensite. As a steel sheet which shows the high strength and in which the hole expansibility is intended to be improved by producing the steel which has the composite structure, Patent Document 1 discloses a high strength hot rolled steel sheet where a fraction of the microstructure of the steel which consists of the mixed structure of ferrite, martensite, and residual austenite is appropriately controlled. The characteristic values of the steel sheet which is obtained by the technique are tensile strength of 590 MPa or more and hole expanding ratio of approximately 50%.
Patent Document 2 discloses a high strength hot rolled steel sheet which consists of a mixed structure of ferrite and martensite, which is precipitation-strengthened by carbides of Ti or Nb. The characteristic values of the steel sheet which is obtained by the disclosed technique are tensile strength of 780 MPa or more and hole expanding ratio of approximately 50%.
However, for example, for steel sheets which are used as suspension members or the like of the automobile, a steel sheet which shows excellent coexistence of the tensile strength with the hole expansibility, such as tensile strength of 590 MPa or more and hole expanding ratio of 60% or more as the characteristic values thereof, is anticipated. In particular, a steel sheet which has hole expanding ratio of 90% or more when the tensile strength is 590 MPa to less than 780 MPa and which has hole expanding ratio of 60% or more when the tensile strength is 780 MPa to 980 MPa is anticipated.
In addition, since the variation of each measurement of the hole expanding ratio is comparatively large, it is necessary to reduce a standard deviation σ of the hole expanding ratio which is an index representing the variation, in addition to an average λave of the hole expanding ratio in order to improve the hole expansibility. As described above, in the steel sheets which are used as the suspension members of the automobiles, a steel sheet which has preferably standard deviation σ of the hole expanding ratio of 15% or less and which has more preferably standard deviation σ of the hole expanding ratio is 10% or less is anticipated.
In addition, for example, in a case where the automobile drives over a curb and a strong impact load is applied to the suspension parts, fracture may occur from a punching surface of the suspension parts as a starting point. In particular, since the notch sensitivity increases with an increase in the strength of the steel sheet, the fracture from the punching end face are strongly concerned. For this reason, for the steel sheets which are used as structural materials of the suspension parts or the like, it is necessary to improve the fracture properties. As indices representing the fracture properties, resistance of crack initiation Jc (unit: J/m2) and resistance of crack propagation T. M. (tearing modulus) (unit: J/m3) which are the characteristic values which are obtained by a three point bending test with notch, and fracture appearance transition temperature vTrs (unit: ° C.) and Charpy absorbed energy E (unit: J) which are obtained by a Charpy impact test may be exemplified. The resistance of crack initiation Jc represents the resistance to the initiation of cracks (the start of fracture) from the steel sheet which composes the structural material when the impact load is applied. On the other hand, the resistance of crack propagation T. M. represents the resistance to large-scale fracture (the propagation of fracture) of the steel sheet which composes the structural material. In order not to decrease the safety of the structural material when the impact load is applied, it is important to improve both of the resistances.
Conventionally, techniques, in which the characteristic values, in particular, the resistance of crack initiation Jc and the resistance of crack propagation T. M. which are characteristic values obtained by the three point bending test with notch intend to be improved, have not been disclosed.
In addition, repeated stress is applied to the suspension parts for the automobile. Therefore, since occurrence of the fatigue fracture is concerned, excellent fatigue properties are also required for the steel sheets which are used as structural materials such as suspension parts.